The present invention relates to a horizontal furnace for processing semiconductor devices such as a semiconductor integrated circuit (IC). More particularly, it relates to a high temperature horizontal furnace having a suspension cantilever which supports workpieces, such as wafers, in a furnace tube during all stages of the process including loading, unloading and processing of the wafers.
Processes used in the fabrication of semiconductor devices such ICs include a lot of physical and chemical processes such as a diffusion process for the formation of doped diffusion regions in the wafers, an oxidation process for forming oxide layers on the wafers, a chemical vapor deposition (CVD) process for formation of epitaxial layers, etc., wherein high temperature heat treatments are employed. The heat treatment temperature often exceeds 1000.degree. C. While the furnace type currently most commonly used in a mass-production line for ICs is a horizontal furnace, vertical type furnaces also may be used. In order to improve the productivity and reliability of the heat treatments, large heat-treatment capacity, uniform temperature distribution and dust-free cleanliness of the horizontal furnace are required.
Generally, a dust-free system is essential for the production of semiconductor devices. With respect to horizontal furnaces, there has been a problem in connection with dust mainly comprised of particles generated by friction between the quartz boats and the quartz furnace tube during loading and unloading of the wafers. The particles are comprised mainly of quartz, being derived from the wear generated by the quartz-to-quartz friction between quartzwares, boats and furnace tubes, etc. In addition, residual particle materials are deposited on the inner walls of the quartz tube during heat treatments of CVD processes, for example, which cause additional dust particles in the furnace.
FIG. 1 is a cross-sectional view of a prior art horizontal furnace, illustrating its structure and the loading of wafers to be heat treated. Wafers A are stacked in a plurality of quartz baskets B and accommodated on a quartz boat C. The wafers A are loaded into the furnace by pushing the boat C into the tube 1 in the direction of the arrow a, using a quartz bar 4 having a hooking notch at its end. Similarly, the wafers A are unloaded by pulling the quartz boat C out of the tube 1 with the bar 4. The inlet of the furnace tube 1 is covered by a quartz cap 2. Furnace gas such as nitrogen is introduced through a gas inlet pipe 1a and flows through the tube 1 and out through an outlet pipe 1b.
The furnace tube 1 has heaters 3, shown in dot-dash lines in FIG. 1, disposed outside the tube wall, and is surrounded by a layer of heat insulation material (not shown) such as alumina. A temperature equalizing tube (not shown) of high thermal conductivity ceramic material is arranged outside the tube 1 to obtain a uniform axial temperature distribution. With the horizontal furnace of this type, the generation of particles is inevitable during the loading and unloading of the wafers as described above. In addition, sticking between the quartz tube 1 and quartz boat C tends to occur in a treatment at high temperatures such as approximately 1100.degree. C.
In order to eliminate the generation of particles as described above, a suspension cantilever loading system has been used, wherein wafers stacked in baskets B are sustained inside the furnace tube 1 during all the stages of the heating process including loading, unloading and heat processing. With this system, the wafers A, baskets B and the suspending means never touch the inner wall of the furnace tube 1, thus eliminating the generation of particles. FIG. 2 is a cross-sectional view illustrating a prior art horizontal furnace having a suspension cantilever loading system extending along the longitudinal axis of the furnace tube 1. For loading or unloading wafers from the horizontal furnace, a suspension cantilever means 14 carrying baskets B stacked with the wafers is forwarded into or pulled out from the center of the furnace tube 1 by a cantilever drive mechanism 15. The upper portion of the cantilever 14 has a boat-like shape suitable for accommodating the baskets B. A cap 12 of the furnace tube 1 has a special opening 12a allowing the passage of the supporting portion of the cantilever.
FIG. 3 is a perspective view of a prior art suspension cantilever system. By automatically controlling the operation of the cantilever driver 15, the loading and unloading of the wafers A can be performed according to a predetermined schedule to reduce any undesirable thermal shock to the wafers, which might cause damage to them. Although loading and unloading without contacting the inner wall of the tube 1 can be accomplished by this cantilever suspension system, a problem of deformation of the cantilever 14 arises during high temperature heat treatment. In order to minimize the deformation of the suspension cantilever 14, supports of heat-resistive material such as pure alumina (Al.sub.2 O.sub.3) or high-grade silicon carbide are used. The supports are sheathed with pure fused quartz in order to be easily cleaned. However, the strength of these materials is limited at high temperatures.
To overcome the problem of the high temperature deformation, a "soft-landing," i.e., descending vertically without any shock, of the baskets B onto the inner wall of the tube 1 during the heat processing can be carried out by controlling the cantilever driver 15. After the soft-landing of the baskets B, the cantilever 14 is retracted from the furnace tube 1 so as not to be exposed to heating. However, this requires a more complicated mechanism of the driver 15, and contact between the baskets B and the inner wall of the furnace tube 1 still generates particles.